Conventional methods of sulfurizing a cast iron material, to form a sulfurized layer (composed mainly of iron sulfide) on its surface, include (a) a high-temperature sulfurizing procedure in which the material is heated to about 570.degree. C. using a suitable combination of, e.g., sodium cyanide (NaCN), potassium ferrocyanide [K.sub.4 Fe(CN).sub.6 ], sodium sulfite (Na.sub.2 SO.sub.3), sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.3), ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ] and sodium sulfate (Na.sub.2 SO.sub.4), and (b) a low-temperature sulfurizing procedure wherein the sulfurization is carried out at about 150.degree. C. However, products of both of these procedures have only a thin (thickness of less than 10 microns) sulfide layer or a layer containing both a sulfide and a nitride. For this reason, such resulting layer has insufficient corrosion resistance to molten aluminum, notwithstanding its good wear resistance and seizure resistance. It is also difficult to obtain the desired sulfurized layer consistently, and these methods lack working stability.
A conventional sulfurizing method, involving pretreating the material with an iodine-alcohol solution, can result in a sulfurized layer having a thickness of as much as several tens of microns and superior resistance to molten aluminum. However, the use of iodine is undesirable from the standpoint of pollution control; and, since alcohol is flammable, its use is undesirable from the viewpoint of safety in operation.